Closing of bags of thermoplastic material, such as polyethylene, is commonly accomplished by sealing the confronting bag side walls together. Such sealing is commonly done by a band sealer which carries the bag between confronting runs of adjacent endless sealing bands of steel or similar material. Heat is transmitted through such bands to the bags for accomplishing sealing, and similarly, heat is withdrawn from the polyethylene through the bands subsequent to the sealing.
In order to properly control and propel the bag along the sealing bands, confronting runs of chains are located immediately beneath the bands and are used to grip and convey the bag along the sealing bands.
The bands and chains are driven from a single motor and the speed of the confronting and adjacent runs of bands and chains must be carefully controlled in order that there is no speed differential between various portions of the bag. If such speed differential occurred between any of the bands or chains, deformation of the bag by tearing or stretching would occur, and this would disrupt the sealing process.
The area beneath the chains and bands must be completely unobstructed so that the bags can freely pass through this space. Because of this need for obstruction-free space beneath the bands and chains, the driving mechanism for the bands and chains has been located above the bands and chains.
It has been common practice to drive the sealing bands by applying driving power to the end wheel about which the bands are trained. Each of these wheels have been driven by a shaft extending downwardly to the respective wheels from pinions which carefully coordinate the speed of the two wheels and confronting bands. Similarly, the bag-carrying chains are driven by sprockets at one end of the chains and the speed of such sprockets is coordinated with respect to each other by meshed pinions on the drive shaft of the respective sprockets. The power from the motor is supplied to these several shafts by additional chains and sprockets.
Because the sealing bands are subjected to heating to sealing temperatures and rapid movement around the wheels, warpage and breakage of such bands occurs from time to time, necessitating replacement of the bands. Replacement of such bands has been accomplished only by complete removal of the band wheel mounting and driving shafts, which necessarily involves the work of a mechanic in disassembling and reassembling parts of the machine, and of course the subsequent realignment.